This invention is related to a fuel injector pump for a diesel engine, and particularly to a fuel injector pump having an accumulator for preventing the formation of harmful fuel vapor in the pump passages.
The invention contemplates an anti-vapor improvement for an existing fuel injector pump. This pre-existing pump comprises a pump housing equipped with a solenoid-operated control valve for timing the flow of pressurized fuel to a fuel injector at an engine cylinder, whereby a desired quantity of fuel is injected into the cylinder at the desired point for efficient engine performance.
The fuel injector pump comprises a relief chamber connected to the pump fuel outlet passage, such that during the initial portion of the pumping stroke some, or all, of the pressurized fuel is directed into the relief chamber, rather than going to the fuel injector. Such fuel flows from the relief chamber to a fuel return means leading back to the fuel supply. At some point in the pumping stroke the solenoid operator for the control valve is energized to cause the valve to interrupt the connection between the fuel outlet passage and the relief chamber, such that pumping chamber output is directed into the fuel outlet passage leading to the associated fuel injector.
With the described pump, the quantity of fuel delivered to the fuel injector is determined by the duration of the electrical signal sent to the solenoid operator for the control valve. The timing of the injection is determined by the timing of the electrical signal.
As noted above, there is a period at the beginning of the pumping stroke when all, or most, of the pressurized fuel is diverted from the fuel outlet passage through the relief chamber to the fuel return means. The fuel return means is essentially at zero pressure, such that the pressurized fuel undergoes a substantial pressure drop as it flows from the outlet passage through the relief chamber; the fuel velocity is relatively high in the relief chamber. At the instant when the control valve interrupts the connection between the outlet passage and the relief chamber the fast-flowing fuel in the relief chamber tends to create a vacuum condition in the relief chamber by the inertia effect. The fuel tends to vaporize. Also a relatively large pressure spike can be generated at the control valve.
Vaporization of fuel can cause damage inside the pump by a phenomenon known as cavitation erosion. Large pressure spikes can contribute to fuel leakage failure.
The present invention is directed to a mechanism for preventing, or minimizing, the undesired fuel vaporization and pressure spikes. Under the present invention, a flow restrictor orifice is provided between the fuel relief chamber and the depressurized fuel return means (passage). The orifice materially slows fuel velocity through the relief chamber so that when the control valve interrupts the connection between the outlet passage and the relief chamber the inertia forces in the relief chamber are reduced to a point where there is essentially no vaporization of the fuel flowing through the relief chamber. The orifice similarly affects the short duration flow out of the control valve at the end of injection.
The restrictor orifice offers the further advantage of pressurizing the fuel in the relief chamber. While the control valve is in the process of closing the relief chamber the pressurized fuel in the relief chamber can absorb any pressure spike being generated in the outlet passage proximate to the valve opening. The pressurized relief chamber acts as an accumulator to absorb the pressure spike before it can develop to harmful proportions. The orifice protects the depressurized fuel return means from harmful pressure spikes.
The solenoid-operated control valve used on the injector pump includes a solenoid armature located in an armature cavity in the pump housing. The control valve poppet is connected to the armature by a slidable plunger that extends through the fuel outlet passage. During operation of the fuel injector some pressurized fuel can leak from the outlet passage into the armature cavity via the clearance between the valve plunger and its guideway. The armature cavity is connected to a low pressure fuel inlet passage in order to supply fuel to the pumping chamber.
The pressurized fuel flowing through the armature cavity can vaporize for essentially the same reasons as previously discussed in connection with flow through the relief chamber. Under the present invention, a second flow restrictor orifice is provided between the armature cavity and the low pressure inlet passage. This second flow restrictor orifice prevents undesired vaporization of any leakage fuel in the armature cavity.
Further features of the invention will be apparent from the attached drawing and description of an illustrative embodiment of the invention.